Before the Flood Arrives

New NASA Study May Improve Future River-Observing Satellites

River floods are one of the most common and devastating of
Earth's natural disasters. In the past decade, deluges from rivers have killed
thousands of people every year around the world and caused losses on the order
of tens of billions of U.S. dollars annually. Climate change, which is
projected to increase precipitation in certain areas of the planet, might make
river floods in these places more frequent and severe in the coming decades.

Now, a new study led by researchers at NASA's Jet Propulsion
Laboratory in Pasadena, California, analyzes what it would take for river-observing
satellitesto become an even more
useful tool to mitigate flood damage and improve reservoir management globally in
near real-time.

"Early flood warning systems traditionally depend on gauge
networks that detect floods farther up the river, but gauge data are becoming
more and more scarce," said George Allen, lead author of the new research
and a hydrologist at JPL. "Our study shows that there's room for satellites to help
fill in the gap. But for satellites to inform real-time flood mitigation, they
have to provide data to water managers within a sufficiently short lag time."

River floods occur when a channel fills with water beyond the
capacity of its banks, normally due to heavy rainfall. The flood travels along the
course of the river as a wave, moving downstream faster than the water itself.
Several satellite missions have been able to detect floods as sudden changes in
the height or width of river waters. Once a flood is observed, it is relatively easy to predict accurately
how it will move down the river. This information is extremely useful in early
flood warning systems and other real-time river management applications.

To study the speed at which floods propagate through the
planet's rivers, Allen and his colleagues ran a simple numerical model of flow
waves that used information such as the width, slope, depth and roughness --
the amount of friction water experiences when traveling along a river -- of
rivers worldwide. After analyzing wave speeds through 11 million miles (17.7
million kilometers) of rivers around the planet, the researchers found that
flood waves traveling at their maximum speed take a median time of three days to
reach the next downstream dam, four days to arrive to the next downstream city and
six days to exit the river system entirely.

The team compared their model's results with discharge
records from more than 20,000 U.S. Geological Survey gauge stations along
around 40,000 miles (64,400 kilometers) of varied river systems in the United
States. They found that the model estimated faster wave speeds than the gauge
data showed.

"That was expected, based on the fact that we're modeling waves
moving at maximum speeds, whereas the gauge data are looking at all types of
wave speeds: low speeds, high speeds, everything in between," Allen said. "In
this way, our study estimates a worst-case-scenario of how fast floods can move
down rivers."

The scientists then used their wave speed findings to
calculate data latency -- how quickly satellite data should be downloaded,
processed and made available to the public to be useful for flood early warning
systems and other real-time flood mitigation strategies, as well as reservoir
management. In particular, they focused on future data from NASA's upcoming
Surface Water and Ocean Topography (SWOT) mission. SWOT, scheduled to launch in
2021, is specifically designed to observe rivers. That's because it has a
repeat orbit of 21 days and will be able to detect flood waves, particularly in
higher-latitude large rivers. The researchers found that making SWOT data
available within days after being acquired by the spacecraft could be useful
for real-time flood mitigation. Compared to past or
current satellites providing river and flood information, SWOT will provide
never-before-seen maps of river height, allowing for more reliable prediction of flood timing and magnitude.

If the data were to be processed in two days or less, Allen's
team calculated, it would be ready for emergency managers before at least two-thirds
of observed waves reached the next downstream city. For dams, the quick
turnaround of satellite measurements would give advance notice to downstream
reservoirs in at least half of the cases when SWOT detects a flood wave.

"There is a trade-off between data latency and data quality,"
said Cédric David of JPL, who directed the new study and is a member of SWOT's
science team. "So, do we want to wait to get the best data possible, or do we
want to get a rough version of what's going on now, so we can provide
actionable information? As we prepare for new satellite missions like SWOT,
that's when we start asking these types of questions."

Satellite data that could inform flood early warning systems
would be particularly useful for developing nations, where either there are
insufficient river gauges or countries do not share gauge data with their
downstream neighbors, Allen said.

Results of the study
are published in the journal Geophysical Research Letters.